1. Field of the Invention
The present invention relates to variable displacement compressors that are used in vehicle air conditioners and to a method for controlling the compressors. More particularly, the present invention relates to a variable displacement compressor equipped with a displacement control valve that controls the inclination of a swash plate and to a method for controlling the compressor.
2. Description of the Related Art
A typical variable displacement compressor has a cam plate tiltably supported on a rotary shaft. The inclination of the cam plate is controlled based on the difference between the pressure in a crank chamber and the pressure in the cylinder bores. The stroke of each piston is varied in accordance with the inclination of the cam plate. The displacement of the compressor is varied, accordingly. The compressor is provided with a discharge chamber that is connected to the crank chamber by a supply passage. A displacement control valve is located in the supply passage. The control valve controls the flow rate of refrigerant gas from the discharge chamber to the crank chamber thereby controlling the pressure in the crank chamber. Accordingly, the difference between the pressure in the crank chamber and the pressure in the cylinder bores is varied.
The control valve includes a valve body for controlling the opening of the supply passage and a transmission mechanism for transmitting changes in the suction pressure to the valve body. The valve body is selectively moved in a direction opening the supply passage and in a direction closing the passage. The transmission mechanism changes the position of the valve body in accordance with the suction pressure acting thereon for changing the opening of the supply passage. The control valve includes a solenoid having a steel core and a plunger. The plunger is selectively moved toward and away from the core. Applying electrical current to the solenoid generates an attractive force between the core and the plunger. The magnitude of the force varies in accordance with the value of the current. The force moves the valve body in one of the moving directions. Therefore, the required magnitude of suction pressure for moving the valve body in a direction opening or in a direction closing the supply passage is changed in accordance with the value of current supplied to the solenoid. In other words, even if the suction pressure is constant, the opening of the supply passage is changed in accordance with changes in the value of the current supplied to the solenoid.
Applying a constant direct current to the solenoid creates a constant attractive force between the fixed core and the plunger. The magnitude of the force is proportional to the applied current value. If the suction pressure is constant, the constant attractive force allows the plunger to remain at a substantially static position. In this state, if the current value to the solenoid is changed, the plunger is moved from the substantially static position. The plunger is slidably retained in the housing of the housing. Thus, frictional force is generated between the plunger and the housing. The maximum static frictional resistance between the plunger and the housing is greater than the kinetic frictional resistance. Moving a static plunger thus requires a force that is greater than the maximum static frictional resistance force. Therefore, the attractive force between the core and the plunger needs to be relatively large, which is accomplished by sending a relatively large current to the solenoid or by enlarging the size of the solenoid. This increases the power consumption of the solenoid.
A greater power consumption increases load on auxiliary components such as the alternator. This results in a greater load on an external drive source such as an engine that drives the compressor and the auxiliary components. Since the space for a compressor in an engine compartment is relatively small, the compressor must be compact. However, increasing the size of the solenoid enlarges the compressor.
Variable displacement compressors often have a rotary shaft directly connected to an external drive source such as an engine without an electromagnetic clutch located in between. In such a clutchless system, the compressor is operated with the minimum displacement even if refrigeration is not necessary. Therefore, the load on the external drive source must be minimized in a clutchless system. Since it has no electromagnetic clutch, a clutchless system consumes relatively little electricity. This reduces the load on the auxiliary components and the external drive source. For further reducing the power consumption, the value of current supplied to the solenoid in the control valve must be decreased. However, this results in a narrower range of current values that can be supplied to the solenoid. Altering the current value to the solenoid only slightly does not generate a force greater than the maximum static frictional resistance force of the static plunger and does not move the plunger. If the range of possible changes in current value to the solenoid is narrow, it is difficult to finely and accurately control the control valve.
Supplying current to a solenoid warms the solenoid. Temperature changes in the solenoid vary the electrical resistance of the solenoid. As a result, the actual current value in the solenoid deviates from a target current value. This prevents the control valve from being accurately controlled.